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Abstract
Spatial patterns of plant disease can provide insights into important epidemiological processes such as sources of inoculum, mechanisms of propagule dissemination, and reproductive strategies of the pathogen population. The goal of this study was to characterize spatial patterns of brown rot (caused by Monilinia spp.) and of the associated fungal genotypes in complex stone fruit canopies. An electromagnetic digitizer was used to map and georeference symptoms within individual tree canopies, and three-dimensional methods of spatial statistics were applied to analyze the resultant data points for aggregation and association patterns. The approach was tested and validated in eight sour cherry trees affected by M. laxa, and applied subsequently in a 3-year study on peach to characterize spatio-temporal development of pre-harvest brown rot, caused by M. fructicola, in 13 trees of different maturity classes. Disease aggregation correlated negatively with disease incidence (r = 0.653, P < 0.0001), showing that trees with higher brown rot incidence had lower aggregation of affected fruit. Significant disease aggregation was most pronounced for early-maturing cultivars and/or early in the epidemic. This is consistent with a greater importance of localized, within-tree sources of inoculum at the beginning of the epidemic. To complement these results with genotypic information about the associated pathogen isolates, 16 microsatellite markers were developed and applied to examine the fine-scale genetic structure of M. fructicola in six of the 13 trees, whereby isolates from every brown rot symptom in each tree were genotyped. All populations (65 to 173 isolates per tree) showed high genetic and genotypic diversity. The percentage of unique multilocus haplotypes within trees was greater for blossom blight isolates than for fruit rot isolates, indicating a greater contribution of clonal reproduction during the latter phase. Spatial genetic structure was observed among fruit rot isolates, with all six populations showing positive and significant autocorrelation up to 0.37 and/or 0.73 m. Despite high levels of within-tree pathogen diversity, the contribution of locally available inoculum is likely the main factor in generating the observed fine-scale spatial patterns of disease and pathogen genotypes within individual trees.